The electrical systems in residential, commercial and industrial applications usually include a panelboard for receiving electrical power from a utility source. The power is then routed through overcurrent protection devices to designated branch circuits supplying one or more loads. These overcurrent devices are typically circuit interrupters such as circuit breakers and fuses which are designed to interrupt the electrical current if the limits of the conductors supplying the loads are surpassed. Interruption of the circuit reduces the risk of injury or the potential of property damage from a resulting fire.
Circuit breakers are a preferred type of circuit interrupter because a resetting mechanism allows their reuse. Typically, circuit breakers interrupt an electric circuit due to a trip condition such as a current overload or ground fault. The current overload condition results when a current exceeds the continuous rating of the breaker for a time interval determined by the trip current. The ground fault trip condition is created by an imbalance of currents flowing between a line conductor and a neutral conductor such as a grounded conductor, a person causing a current path to ground, or an arcing fault to ground.
An example of a ground fault interrupter is a fast acting circuit breaker that disconnects equipment from the power line when some current returns to the source through a ground path. Under normal circumstances all current is supplied and returned within the power conductors. But if a fault occurs and leaks some current to ground, then the ground-fault circuit interrupter (GFCI) will sense the difference in current in the power conductors. If the fault level exceeds the trip level of the GFCI, then the circuit will be disconnected. The trip level for protection of personnel is usually in the range of about 4 to 6 mA. The trip level for the protection of equipment is usually about 30 mA.
GFCI and other equipment often use solenoid coils for protection against electrical transients, particularly when electrical voltage clamps like metal oxide varistors (MOVs), zener diodes or spark gaps are used in the circuit. The coil must absorb the transient surge of both voltage and electrical energy in a short period of time, typically on the microsecond order. Should the transient voltage breakdown the coil, it could endanger other components of the circuit.
The ability of the coil to withstand voltage depends on the insulation between the coil's windings. In the conventional manufacture of a coil winding on a bobbin, a lead wire extends down along the side of the bobbin to the surface of the bobbin's core. Tape is usually placed over this lead wire for electrical insulation and to retain the lead wire as the coil is being wound. The lead wire is usually the area of initial voltage breakdown, however, because it extends from the top to the bottom of the layered winding's side.
There are further complications to improving the ability of the coil to withstand a transient voltage surge. To achieve a higher voltage rating, either the distance between the winding layers must be increased or a barrier must be inserted. As devices which carry the coil become increasingly smaller themselves, however, there is a need to achieve these protective characteristics in a more compact design.
In view of the increasing size restriction for coils and their devices, there is a need for a coil assembly with the ability to absorb transient voltage surges in a more compact design. There is another need for an inexpensively manufactured coil which can more effectively insulate the lead wire from the remainder of the coil winding to improve its ability to absorb transient voltage surges.